The main concern with this continuing rise is that it forecasts a similar increase in the rate of type 2 diabetes, according to researcher Dr. Gary Liguori, an assistant professor of health, nutrition and exercise sciences at North Dakota State University in Fargo.

The new study found that 34 percent of U.S. adults in government health surveys conducted between 1999 and 2006 had metabolic syndrome -- up from 29 percent in similar surveys done between 1988 and 1994.

The researchers estimate that about 50 million U.S. adults had metabolic syndrome in 1990 and 64 million had it in 2000. The number of Americans with metabolic syndrome between 1999 and 2006 was probably about 68 million, according to the researchers.

In general, the researchers found, the rise in metabolic syndrome was primarily due to growing rates of abdominal obesity and high blood pressure.

What's more, the study found, the most significant increases in metabolic syndrome were occurring in women - and mostly in younger women, regardless of whether they were black or white. For example, among all young women between the ages of 20 and 39, 11 percent had metabolic syndrome in 1988-1994, whereas 17 percent had it in 1999-2006.

The researchers also saw an increase in metabolic syndrome among men, although not as dramatic as for women. And the increase in men wasn't concentrated in any particular age group.

In Mexican-Americans, there wasn't much of a change over time, but that doesn't mean they were in good shape overall. On the contrary, in 1999-2006, 37 percent of Mexican-American men and 43 percent of the women had metabolic syndrome - the largest percentages for any subgroup.

For individuals, the findings underscore the importance of a healthy lifestyle -- which includes maintaining a normal weight through a balanced diet and regular exercise, Liguori said in an interview.

From a public-health standpoint, he said, the results point to a continuing need to rein in childhood obesity rates. He noted that the increase in metabolic syndrome among younger adults may, at least in part, reflect the rise in childhood obesity that began in the 1980s.

The study, published in the journal Diabetes Care, adds to concerns about the current and future toll of type 2 diabetes in the U.S.

In a study reported last year in the same journal, researchers estimated that by 2034, the number of Americans with diabetes will nearly double -- from 24 million to 44 million -- and direct medical costs for the disease will triple. (See Reuters Health story of Nov 27, 2009.)

They forecast that the annual price tag could reach $336 billion over the next 25 years -- assuming there is no change in obesity rates and diabetes prevention. If the obesity situation worsens, the researchers said their figures could be underestimates.

For the current study, Liguori and his colleague Dr. Arupendra Mozumdar examined data from two waves of the National Health and Nutrition Examination Survey (often referred to as "NHANES"), an ongoing series of government studies that includes interviews and physical exams of nationally representative samples of U.S. adults.

Because obese young adults often were overweight as children or teens, these findings emphasize the importance of preventing childhood obesity in the first place, Liguori said.

Exactly how to most effectively curb childhood obesity is not clear, but experts generally call for a range of efforts -- from improving the quality of food sold in schools, to parents' limiting kids' TV and computer time, to incentives for vendors to offer more fresh produce and other high-quality foods in low-income neighborhoods.